GaN and related materials continue to grow in importance for optical and electronic devices. As in other semiconductor systems, epitaxial growth of GaN ideally occurs on GaN substrates cut from bulk GaN single crystals. Bulk crystal growth of GaN, however, requires extremely high pressure to maintain the nitrogen content in the crystal, rendering bulk growth extremely difficult. For this reason, the high volume production of large size, bulk GaN is improbable in the near future and the search for alternative substrates continues.
Two of the main factors associated with substrate choice are cost and resulting GaN epilayer quality. Silicon is increasingly being used as a substrate for GaN deposition because Si substrates are available at comparatively low cost, high quality, large area, and large quantity, thus presenting many manufacturing advantages over other available substrates for GaN, such as sapphire and SiC.
The disadvantages of Si as a substrate for GaN heteroepitaxy include an a-plane +20.5% misfit which led to the conclusion that growth of GaN directly on silicon was unfeasible. Moreover, the thermal expansion misfit between GaN (5.6×10−6 K−1) and Si (6.2×10−6 K−1) of 9.6% can lead to cracking upon cooling in films grown at high temperature, and, at elevated temperature, melt-back etching between Ga and the Si substrate during the initial stages of growth or at stress is known to induce cracks that form in GaN films during GaN deposition.
Traditionally these issues trigger polycrystalline GaN growth on Si substrates. Typically, thin AlN buffer layers are used to absorb the lattice mismatch between the GaN film and the Si substrate. The subsequent deposition of GaN introduces significant strain into the structure due the large lattice mismatch along with the resultant high density of defects that introduce additional tensile stress into the film. This tensile stress is exacerbated during cool down from growth temperature with macro-crack formation customary for GaN films thicker than 1 μm.
To overcome GaN cracking problems, different techniques have been used including use of multiple AlN interlayers, AlGaN graded layers, patterned Si, and in situ SiN masking (non-uniform deposition). These methods were reported to provide some decrease in bowing and cracking, but no method successfully produced crack-free thick (e.g. >10 μm) GaN films likely because there still remains excessive tensile stress, as well as strong cohesion between GaN (or AlN buffer layer) and Si. Although ˜7 μm thick crack-free GaN on Si has been reported by incorporating multiple AlN interlayers, the maximum thickness of a commercially available crack-free GaN layer on Si is about 1 μm.
Cracks can be generated during growth or cooling due to the excess tensile stress caused by large lattice and thermal expansion differences. It has been observed by the present Inventors that the cracks penetrate through the Si substrate and separation occurs inside the Si substrate. The strong cohesion between GaN and Si (or AlN and Si in GaN/AlGaN/AlN/Si template case), as well as the brittleness of Si, are responsible for cracking to take place in pre interior of the Si wafer. The bond strength of Si—Si is 7 eV which is lower than and the Ga—N (8.9 eV) or Al—N (11.5 eV) and Si—N (10.5 eV). The bond strength of Si—Si is the weakest. The nano-indentation hardnesses of the GaN, AlN, and Si are 20, 18 and 14 GPa, respectively. Therefore, the cracking penetration to the Si substrate observed by the present Inventors was expected. This brittleness of Si added with the large tensile stress created by the lattice mismatch and thermal expansion differences makes the growth of crack-free GaN on Si even more challenging.